In the laying of underground cables for telephones, power transmissions and the like, wires and cables are frequently led through a number of parallel ducts. Although it is possible to use asbestos-cement, ceramic, steel or pitch-impregnated fiber pipe for this purpose, the most popular material is duct made by extruding plastic material. Twenty-foot lengths of the plastic duct can be cemented end-to-end above ground and a number of the parallel strings thus formed bundled together at spaced intervals by tight flexible bands. The bundles thus formed can then be lowered into a prepared trench and the trench backfilled.
If parallel strings of flexible or semiflexible duct lengths are bundled together so that adjacent ducts are in mutual tangential contact throughout their lengths, deflection of the individual ducts can be minimized. If the ducts at the same time can be maintained in a square or rectangular pattern, the depth and width of the trench that is required to accommodate the bundled duct strings will be at an optimum.
A primary object of this invention, therefore, is to facilitate positioning and maintaining bundled ducts in a square or rectangular cross-sectional pattern during and after the operation of burying them in a trench or positioning them in an above-ground installation. Still another object of this invention is to provide a means for positioning subterranean bundled ducts so that each duct throughout its length is in tangential contact with adjacent ducts, thereby distributing the earth load in a downward, outward and even manner.
I am aware that U.S. Pat. Nos. 1,799,673 and 1,868,881 disclose bundles of ducts arranged in square arrays. In these patents the ducts in each bundle are disclosed to be separated from each other by four-sided spacers. In cross section each side of each spacer is concave and describes a circular arc of the same radius as the ducts making up the bundle. If a bundle of ducts is assembled incorporating the spacers of U.S. Pat. Nos. 1,799,673 and 1,868,881 and installed below ground level each individual duct is subjected over its entire surface to the burden and pressures of the overlying and surrounding backfill material, thus making each duct more susceptible to deformation and deflection. There is also a concentration of forces at the areas of contact between the spacers and ducts. This concentration of forces can lead to additional deformation and crushing of the duct at the areas of contact. This crushing and displacement of the individual ducts occurs because of the lack of continuous longitudinal tangential contact between adjacent ducts. If, however, a below-ground installation of bundled ducts is made so that each duct is maintained in continuous longitudinal tangential contact with adjacent ducts and/or backfill material, load forces tend to be distributed evenly and equally throughout the entire duct bundle.